The invention relates to a belt drive system having a tensioner with a damping mechanism. More particularly, the invention relates to a tensioner with a torsion spring that biases the position of a pivot-arm to which a belt engaging pulley is rotatably mounted. The tensioner of the invention with its damping mechanism is particularly useful in controlling tension of a micro-V belt of a front end accessory drive for automotive engine applications.
A mechanical tensioner is used to automatically control the tension of a belt of a front end accessory drive for automotive engine applications. Such a tensioner has a pivot-arm that rotates about a pivot secured to a base and uses a sleeve-bushing on the pivot to provide a bearing surface for the rotating pivot-arm. Many of such bushings are made of plastic and are subject to wear over the expected life of the tensioner. A torsion spring is often used with one end connected to the pivot-arm and the other end interconnected through the base to bias the position of the pivot-arm and position an attached pulley against a belt. The spring is also used to generate a spring force operative with a damping means that generates a normal force component to a friction sliding surface to inhibit or dampen oscillatory movements of the pivot-arm.
One such tensioner is disclosed in U.S. Pat. No. 4,473,362. The xe2x80x2362 tensioner has a pivot-arm attached to an off-set cylindrical member that supports the pivot-arm and rotates about a pivot secured to a base. Only one torsion spring is used with one end connected to the pivot-arm and the other end connected to the base. A single sleeve-type bushing on the pivot has a bearing surface that supports the cylindrical member. The radial plane of a pulley bearing is off-set in relation to the sleeve-type bushing which introduces a moment or couple as a load which must be carried by the bushing. Such tensioners are sometimes referred as xe2x80x9cZedxe2x80x9d type tensioners because of the off-set of the pulley relative to its support or base structure. Unequal pressure loads introduced into bearing surfaces of the bushing can result in excessive bushing wear and attendant pulley misalignment.
The belt of a drive system utilizing such Zed type tensioners engages the pulley and generates a belt force at the pulley which is transmitted to the cylindrical member (hereinafter hub load). As explained in the xe2x80x2362 patent, the unequal loads to the bushing are reduced by a damping means that generates a normal force component acting in generally the same direction as the hub load. Although the orientation of the belt force with the force component certainly alleviates some of the bushing load and attendant wear problems, it is lacking in some belt drive situations because the normal force component of the damping means is in some cases insufficient to balance against a moment generated by the belt force being off-set from the cylindrical member carrying the hub load, and the single bushing has a tendency to xe2x80x9cbevelxe2x80x9d or xe2x80x9ccrownxe2x80x9d as bearing pressure loads change as the pivot-arm oscillates between clockwise and counter clockwise directions.
A belt tensioner design that solves the xe2x80x9cbevelxe2x80x9d or xe2x80x9ccrownxe2x80x9d on the single bushing is disclosed in U.S. Pat. No. 5,647,813. As explained in the xe2x80x2813 patent, the hub load and normal force component generated by the damping mechanism are carried by at least one or two bushings having two axially spaced-apart bearing surfaces. However, while the xe2x80x2813 tensioner is a good design for solving bushing wear, the use of at least two axially spaced-apart bearing surfaces adds to the overall tensioner size, weight and cost. In addition, the hub load and the normal force component generated by the damping mechanism create a resultant load carried by the bushing which can be too large and cause excessive pivot bushing wear.
In accordance with the invention, a tensioner is provided that is particularly useful in micro-V front end accessory belt drive systems used in automotive applications where minimum tensioner bulk combined with maximum balancing and minimum pivot bushing wear in order to achieve maximum pulley alignment over the expected life of the tensioner is important. The belt tensioner of the invention is of the type with a base. A pivot-arm is attached to a cylindrical member that supports the pivot-arm and rotates about a pivot secured to the base. At least one sleeve-type bushing is positioned on the pivot and includes a bearing surface that supports the cylindrical member. A pulley is attached to the pivot-arm for engaging the belt and receives a belt load that generates a belt force component that is transmitted to the cylindrical member (hub load). A torsion spring having one end connected to the base and another end interconnected to a damping means generates a damping force component acting in the opposite direction as the belt force component. The damping means is mounted substantially between the pivot-arm and the bushing along a plane parallel with the pivot.
An advantage of the invention is that the hub load can be balanced more effectively with substantially less load on the pivot bushing and thereby improve durability. Another advantage of the invention is the reduced size of the bushing without increasing the pressure carried by the bushing and thereby the reduced bulk and cost of the entire tensioner.